The invention relates to a light-emitting device for generating white light, which is composed of a light-emitting diode and a phosphor layer.
Light-emitting diodes are used as signal lights, indicating devices, pilot and warning lamps, light transmitters in light barriers, for optocouplers, IR-remote control systems and optical waveguide transmission systems. They offer many advantages relative to other light-emitting components, such as incandescent lamps. Said light-emitting diodes have a long service life, a great resistance to shocks and vibrations, a good modulation capability into the MHz-range, high packing densities, a great switching circuit compatibility and no inrush currents. They require a low operating voltage and have a low power consumption.
However, it has long been a disadvantage of light-emitting diodes for visible light that not all colors of visible light were available with the same luminous intensity. The efficiency of light-emitting diodes decreases as the wavelength decreases, that is from red, via green, to blue. The brightness of red and green light-emitting diodes was very good and was even substantially further improved by using modern manufacturing methods, however, blue light-emitting diodes exhibited a relatively small luminous intensity. Consequently, using simple means it was impossible to achieve a color-neutral white illumination by a combination of light-emitting diodes.
Theoretically, each color of visible light can be generated from short-wave light, i.e. blue, violet and ultraviolet light. To achieve this, the light-emitting diode, which emits short-wave light, is combined with a suitable phosphor, which converts the short-wave light into the desired color, by absorbing said short-wave light and re-radiating light of the other color in a longer-wavelength range.
White light can be generated, for example, by a blue-emitting light-emitting diode if said light-emitting diode is combined with a phosphor which absorbs blue light, converts it and subsequently emits it as light in the yellow-orange range of the spectrum. Said yellow-orange light mixes with the remaining blue light from the light-emitting diode, and the combination of blue and the complementary color yellow results in white light.
For example JP 08007614 A (Patent Abstracts of Japan) discloses a planar light source for which a light-emitting diode is used, which emits blue light and which is combined with a fluorescent layer of an orange-fluorescent pigment, so that the blue light of the diode is observed as white light. A drawback of this light source is that the chromaticity of the white light is influenced substantially by the small amount of the fluorescent pigment in the fluorescent layer and hence is difficult to control. Only at a high color temperature in the range between 8000 and 8600 K, a good color rendition is obtained. If the color temperature is reduced, also the color-rendering index CRI is reduced substantially.
Therefore, it is an object of the invention to provide a light-emitting device for generating white light, the chromaticity rendition of which can be readily regulated and which has a high color-rendering index.
In accordance with the invention, this object is achieved by. a light-emitting device comprising an UV-diode with a primary emission of 300 nm.ltoreq..lambda..ltoreq.370 nm and a phosphor layer including a combination of a blue-emitting phosphor having an emission band, with 430 nm.ltoreq..lambda.470 nm, a green-emitting phosphor having an emission band, with 525 nm .ltoreq..lambda..ltoreq.570 nm, and a red-emitting, europium-containing phosphor having an emission band, with 600 nm.ltoreq..lambda..ltoreq.630 nm.
The light-emitting device exhibits a high color rendition and, at the same time, a high efficiency because the phosphors absorb the UV-bands with a high efficiency,
This is a continuation of application Ser. No. PCT/IB98/00219, filed Feb. 21, 1998. the quantum efficiency is high, i.e. above 90%, and the half-width value of the emission line is small. The light output is high because in the range above 440 nm and below 650 nm, where the sensitivity of the eyes is low, no light emission takes place.
The white light emitted by the light-emitting device is of a high quality. The color-rendering index CRI is approximately 90 at a color temperature of 4000 K. In addition, the color rendition depends only on the composition of the three phosphors, and not on the relation between converted and unconverted light, and can hence be controlled and regulated in a simple manner.
Within the scope of the invention, it is preferred that the red-emitting phosphor is a line emitter having an emission band with a wavelength maximum of 605 nm.ltoreq..lambda..ltoreq.620 nm.
It is also preferred that the green-emitting phosphor is a line emitter having an emission band with a wavelength maximum of 520 nm.ltoreq..lambda..ltoreq.570 nm.
It is further preferred that the UV-diode is a GaN diode.
Within the scope of the invention it may be preferred that the phosphor layer comprises a blue-emitting phosphor in a quantity x1 of 0&lt;x1&lt;30% by weight, a green-emitting phosphor in a quantity x2 of 20.ltoreq.x2.ltoreq.50% by weight and a red-emitting phosphor in a quantity x3 of 30.ltoreq.x3.ltoreq.70% by weight.
It may also be preferred that the phosphor layer comprises BaMgAl.sub.10 O.sub.17 :Eu as the blue-emitting phosphor, ZnS:Cu as the green-emitting phosphor and Y.sub.2 O.sub.2 S as the red-emitting phosphor.
Within the scope of the invention it is particularly preferred that the phosphor layer comprises a phosphor of the general formula [Eu(diketonate).sub.a X.sub.b1 X'.sub.b2 ] as the red-emitting phosphor, wherein X=pyridine or a monodentate pyridine derivative and X'=2,2'-bipyridine or a 2,2'-bipyridyl derivative and 2a+b.sub.1 +2b.sub.2 =8.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.